In the field of the beverage-producing industry, there is an increasing tendency to use plastics-material containers and PET containers instead of glass bottles. In the manufacture of these containers plastics-material pre-forms are first made available, they are heated and are supplied to an expansion process, so as to obtain the finished plastics-material containers in this way. In this case it is customary in the prior art to let the plastics-material pre-forms run through a heating path inside which they are usually heated with infrared radiation.
In addition, however, it is also known in the prior art to use microwave radiation in order to heat the plastics-material pre-forms. In this case the microwave radiation is produced with a microwave-producing device, such as a magnetron, and is then transmitted by way of a conducting device such as a waveguide to the plastics-material pre-forms to be heated. The microwave energy reaching the pre-forms can be controlled in this case by means of tuning units. The force applied to the plastics-material pre-forms is usually set before the apparatus is put into operation and the apparatus is then operated with this fixed force.
In this case, however, the problem arises that the microwave energy applied to the plastics-material pre-forms can also depend upon the nature of the pre-forms themselves and in this way an equal application of energy into the individual pre-forms or an equal heating is not always guaranteed.
It may be desirable to provide an apparatus and a method which permit a uniform input of energy and a uniform heating of the forms.